Add content to no-std buld.

doc/src/start/builds/no-std.md

@@ -15,7 +15,65 @@ Nightly Required
 Currently, [`no_std`] requires the nightly compiler due to the crates that it uses.
 
 
+Implementation
+--------------
+
+Rhai allocates, so the first thing that must be included in any `no-std` project is
+an allocator crate, such as [`wee_alloc`](https://crates.io/crates/wee_alloc).
+
+Then there is the need to set up proper error/panic handlers.
+The following example uses `panic = "abort"` and `wee_alloc` as the allocator.
+
+```rust
+// Set up for no-std.
+#![no_std]
+
+// The following no-std features are usually needed.
+#![feature(alloc_error_handler, start, core_intrinsics, lang_items, link_cfg)]
+
+// Set up the global allocator.
+extern crate alloc;
+extern crate wee_alloc;
+
+#[global_allocator]
+static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;
+
+// Rust needs a CRT runtime on Windows when compiled with MSVC.
+#[cfg(all(windows, target_env = "msvc"))]
+#[link(name = "msvcrt")]
+#[link(name = "libcmt")]
+extern {}
+
+// Set up panic and error handlers
+#[alloc_error_handler]
+fn err_handler(_: core::alloc::Layout) -> ! {
+    core::intrinsics::abort();
+}
+
+#[panic_handler]
+#[lang = "panic_impl"]
+extern "C" fn rust_begin_panic(_: &core::panic::PanicInfo) -> ! {
+    core::intrinsics::abort();
+}
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+#[no_mangle]
+extern "C" fn rust_eh_register_frames() {}
+
+#[no_mangle]
+extern "C" fn rust_eh_unregister_frames() {}
+
+#[start]
+fn main(_argc: isize, _argv: *const *const u8) -> isize {
+    // ... main program ...
+}
+```
+
+
 Samples
 -------
 
-Check out the [`no-std` sample applications](../examples/rust.md#no-std-samples) for different operating environments.
+Check out the [`no-std` sample applications](../examples/rust.md#no-std-samples)
+for different operating environments.